The invention relates to projection welding of metal sheets.
Projection welding is known for long for welding thin iron sheets above each other, for example in the car manufacturing industry wherein flanges of doors are welded together. A hollow projection is formed in the lower iron sheet by pressing a punch and a die against the sheet before the welding is executed. The sheets are subsequently pressed between two welding electrodes of a welding gun. During welding the projection gradually collapses and a strong structural weld is formed at the place of the projection.
In particular in the car industry there is a growing need for parts that are made of nonferrous metal sheets, in particular sheets of aluminum or magnesium and alloys thereof. These lightweight parts have the same good mechanical properties. However, when the known projection welding process for iron sheets is applied for these metals, there is not obtained a strong structural weld. On the contrary, the projection already collapses before the forming of a weld is started.
In published patent applications prior attempts to projection weld aluminum sheets have been described. However, these initial attempts have not lead to an successful implementation yet, as the applicant has now found out that the critical process parameters to obtain a strong structural weld were not known yet. In general, the described developments to projection weld aluminum sheets were only slight modifications of the process for iron sheets, while it is now found out that a complete different approach is necessary to obtain good results. The developments in the published patent applications are summarized hereafter.
WO 99/03634 in the name of Newcor Inc. discloses the projection welding of aluminum sheets wherein the projection in the lower sheet has the form of a dome. The projection has a height that is about four times the sheet thickness, whereby the projection is very, thin walled and hollow above the main upper surface of the lower sheet. This structurally weak projection will result in a large initial impression or will even collapse immediately after the pressure force is applied, whereby the initial welding contact area is undefined. The welding current is directly obtained from a conventional single phase, 460 Volt, 60 Hz, alternating current mains which is basically sinusoidal in form, wherein welding pulses are generated by breaking the half cycle waves thereof. By applying subsequent, alternating welding pulses onto the undefined initial welding contact, the amount of welding energy that is introduced, in particular at the beginning of the welding cycle, is not under control or to low, whereby a weak weld is obtained.
WO 01/00363 in the name of Newcor Inc. discloses a further development of the process as described here before. The ring shaped projection comprises a hollow upright wall with a curvature. The projection bounds a blind opening that inevitably encloses an amount of air that rapidly expands during the welding cycle. This causes cracks, inclusions and contamination originating from the enclosed air, resulting in a weak weld.
EP 0 102 927 discloses a projection welding process wherein both aluminum sheets are provided with elongate projections that abut each other in a crosswise orientation. Due to the presence of two abutting projections the welding spot will be visible on both sides of the product. This technology can only be applied to low cost utility products such as ducts as disclosed. The upper surface of the projections has a curvature, wherein the initial small welding contact between the crossing projections keeps the initial welding current low. The welding current gradually increases during the collapse of the projections. This implies that the current is dependent on the cross section of the melt only, keeping the control of the welding cycle rather low.
U.S. Pat. No. 4,495,397 discloses the projection welding of aluminum sheets with ring shaped projections, which, causes cracks, inclusions and contamination in the weld originating from the enclosed air. The projections are formed between a punch and a die with straight cold forging faces that are under an angle with each other. The projections have well defined straight side surfaces that are imposed by the die, while the top surface and its curvature are left undefined. This is a straightforward application of the punch and die for welding steel sheets, which will not work for aluminum. During the welding cycle a first pressure force is applied which causes a permanent cold deformation of 8%. The first pressure force is subsequently increased immediately after the welding current has peaked. The welding current is thus controlled by changing the pressure force as leading parameter. This already indicates that the welding current itself is not well under control. Besides that, it is practically impossible to control the pressure force in such way over the short welding cycle, as the pressure force is applied by a mechanically slow system. It seems that the pressure force is changed during the welding cycle in order to compensate other disadvantages somehow.
JP 2002-103056 discloses the projection welding process for aluminum similar to the welding process of U.S. Pat. No. 4,495,397, using ring shaped projections that causes cracks, inclusions and contamination in the welds. The welding process is applied for thick aluminum parts, not for thin sheets.
DE 30 24 333 discloses a cone shaped projection for welding aluminum parts, which is derived from the welding of steel sheets in the same straightforward way as described here before.
DE 100 29 352 describes the projection welding of aluminum sheets with a projection similar to the projection as disclosed in WO 99/03634. The projection has a height that is about two times the sheet thickness, whereby the projection is very thin walled and hollow above the main upper surface of the lower sheet. This projection will suffer from initial impression or will even collapse immediately after the pressure force is applied. The top of the projection has a constant radius and straight side surfaces. This shape comes straight forward from the projection welding of steel sheets. For these reasons this will fail for aluminum.
In the known techniques any oxidation of the objects to be welded needs to be removed prior to welding. In this regard JP06-170549 discloses a process in which the aluminum objects to be welded are reciprocally moved and rotated at the spot of a ring shaped projection for local removal of oxidation prior to welding. However, this cannot be applied, when projection welds are sequentially made between the same objects, as the earlier welds mutually fixate the objects.
It is an object of the present invention to provide a projection and a projection welding method for welding metal sheets of which at least one metal sheet is a non-ferrous metal sheet.